Series transformer.



G. A. BURNHAM.

SERIES TRANSFORMER.

APPLICATION FILED MAR. 31, 1911.

Patented Aug. 20, 1912.

km mgw Wm/mama o 3 Vl/UQ wtoz ym w w MW GEORGE AUGUSTUS BURNHAM, OF CLIFTO'NDALE MASSACHUSETTS. ASSIGNOR SEARS B. CONDIT, JR., OF BROOKLINE, MASSACHUSETTS.

SERIES TRANSFORMER.

Specification of Letters Patent.

Patented Aug. 20, 1912.

Application filed March 31, 1911, Serial No, 618,120.

To all whom it may concern:

Be it known that I, GEORGE A. Br anna M, a citizen of the United States, residing at (.littondale, in the county of Essex and State of Massachusetts, have invented certain new and useful Improvements in Series Transformers, of which the following is a full, clear, and exact specification.

This invention relates to series transformers, the object being to provide a device of this kind which may have its ratio changed to any desirable degree.

Series transformers are usually employed to operate various indicating and recording instruments used on the switchboards of supply or distributing stations, or to furnish energy to actuate trip coils of circuit breakers. In many cases the transformer ratios are such that considerable error of the indicating instruments or calibration of the trip coils is introduced because the curve of transformation ratio is dependent upon many variables peculiarly inherent in current transformers, and because for this reason it is impracticable with ordinary designs to maintain the ratio under light load. There is not range enough in the instru ments themselves to compensate for these errors, and to maintain accuracy in the instruments the only plan available heretofore has been to change their calibration. This is a delicate operation and requires the dismantling of the instruments from the switchboard.

A series or current transformer is widely different from a step-up or step-downtransformer ordinarily employed for lighting and power purposes, in which the primary is supplied by a constant potential circuit and the secondary may be cut in and out of circuit with the translating devices, whenever desired. In the latter type, there is a constant potential primary circuit of many turns. and when the secondary load is cut oft the counter E. M. F. is sufliciently high to permit almost no energy to pass, and the relation of the windings is such that under the constant potential excitation in the primary the volt age at the secondary terminals is practically constant at different loads. In the se ries transformer the secondary potential rises enormously on open circuit, and as the value, if the secondary is not closed to pro- \idc a 'trui'ister of energy the device heats up to such a degree as ultimately to destroy the transformer, while the sccomlary potential rises to a value which t1 c ucutly punctures the insulation. It is therefore important to keep the secondary circuit constantly closed in a series transformer when the device is in action, but there a source of great error arising from a peculiar in ident of series transformer: when at light load the ratio of transtormation is seriously in1- paired. so that the potentials in the secondary circuit are changed and produce erroneous indicatioi'is on the measuring instru ments or other devices supplied. A decrease of potential across the prima "y of a series transformer does not necessarily change the secondary potential. as does ordinary stepup'or stepwlown transfer-Imus consequently, one familiar with the two types would not naturally SIZPPUSS that by ii'itroducingan auxiliary voltage in the secondary of the series transformer, it would materially affect the ratio of transformation, especially as the second ary ircuit is more or less automatic as far as the current goes.

By means of in invention the ratio of transformation may be varied above or be low normal in a very simple manner. I provide the transforn'ier with an iron core having a few turns of wire as a primary circuit, which are connected in series relation to the high tension wires of a distribution system. The secondary coil of generally a greater number of turns links the same magnetic circuit and is entirely insulated from the primary winding. The ra tio of transformation is accomplisl'ied by va'rying the relation of the number of primary turns to the effective number of ondary turns: in the circuit in which the primary winding is connected there may be an extremely high voltage. but in the sec ondary winding the potential is very low. the voltage being on y sutlicient to force the proper amount of ccrrent through the indicating or recording instruments to actuate their moving elements.

l'leretotore,current transformers have had such characteristics that their error increases rapidly at light load and conse uently lll accurate readings in the recording or unlicating devices follow. Furthermore. the he tio of transformation and the slope of the ratio curve is materially changed by variations in the character of the load on the secondary. Thus, if the secondary circuit is changed by the addition or removal of measuring instruments or tripping coils the ratie is destroyed and its curve altered, and the calibrations of the load devices are no longer correct. This is not the case with potential transformers, as a change of secondary load effects a proportional change of primary current to meet the changed conditions.

By my invention the ratio of transformation may be changed at will, and I bring about this desirable result by providing a compensating coil having several brought out from its various turns.

taps g The theory of the current transformer is veryi niuch involved and but little understood, but

the variable results I obtain by the use of' the auxiliary adjustable compensating coil are probably due to the fact that there is an electromotive force introduced in this 0011 tending to increase or decrease the po-@ tentialin the secondary, and consequently vary the ratio of transformation, which, when once ad usted for a given load in constant relation to the secondary terminals, remains constant. I do not wish to confine myself, however, to any particular theory of operation of my transformer. The fact remains that a series transformer constructed in this way will permit a wide range of ratio variation, and furthermore I am able to produce practically any degree of smoothness in the curve and other characteristics never before obtained to my knowledge in the series transformer art. The results I obtain are new in alternating current practice in this art and are of great importance,

to the switchboard engineer, station operators, and consumers of electric energy.

My invention therefore consists in providing a series or current transformer whose ratio of transformation may be varied at will within wide limits and rendered constant" under wide load range with any degree of smoothness that may be required.

The accompanying drawing illustrates my invention, Figures 1 and 2 illustrating the.-

grammatically the nature of my improveoughly insulated from the core.

3 represents" a secondary coitand 4 an adjustable compensating coil provided with a plurality of ta s A connection with any tap may be e ected by providing a lead 6 from the load circuit containing measuring instruments as 7 with a terminal adapted to be inserted in a corresponding socket, one of which is provided for each tap 5. The secondary 3 is also provided with a similar terminal which may be inserted in any tap socket. With this construction any desired amount of the compensating coil 4 may be connected in series relation with the secondary coil so as to buck or boost the secondary potential.

In Fig. 1, I have shown a part of the compensating coil so connected as to buck the secondary E. M. F. In Fig. 3 I have shown a compensating coil connected so that its entire E. M. F. boosts the secondary E.

By means of this construction, any degree of change or transformation ofthe ratio curve may be produced at will by a change of taps, 6 and 7, thus providing a wide range of adjustment. The sheet of ratio curves shown in Fig. 3' was accomplished by only seven turns inthe compensating coil. In this figure the curve A shows the ratio of transformation with the compensating coil omitted; curve B .where the connections were such as to utilize the entire effect of the compensating coil, and connected so as to buck the secondary E. M. F.; curve C when the whole of the compensating coil was connected to'boost, the secondary E. M. F. Between these limits the secondary curve of transformation ratio may be raised or lowered bodily by connecting with the different taps in boosting or bucking relation. It will be noted that the slope of the curve is the same with all forms of connection, tPhat is to say whether the compensating coi is connecting in boosting or bucking relation to the secondary E. M. F. the entire curve is raised or lowered bodily without changing its slope, so with such changes of load as occur in practice, say between forty and one hundred per cent, the curve is perfectly smooth and of uniform potential, so that the calibration of the instruments is in no way affected. Such a result in series transforme'rs'has been greatly sought, but is nowfor the first time available under my invention. The conditions to be met are sui generis to the current transformer and are inapplicable to other forms of transformer, as the reactions and features involved in the two classes of deyices are radically different.

lam aware that taps have sometimes been employed on potential transformers, but with such taps it is a matter of no consequence whether the secondary circuit is open or closed, whereas with series transformers it is imperative that while the transformer is in service the secondary circuit must re-' main closed, otherwise the transformer will be permanently put out of commission. The solution I have applied to provide a variable ratio of transformation which will maintain its smoothness of slope, and therefore insure accuracy of reading and tripping in the in struments supplied, is one that would in no degree be suggested by the practice with step-up and stepdown transformers of the power and lighting art, since in the latter the secondary potential constant under changes of load, whereas with the series transformer a slight change of impedance in the secondary circuit seriously affects the ratio curve and the readings of the instruments.

Having thus described my invention, 1 declare that what I claim as new and desire to secure by Letters Patent is 1. A series transformer having a primary winding adapted to be interposed in series relation to a distribution circuit, a secondary winding of variable number of effective turns, and means for adjusting the secondary winding to accommodate different loads and maintain the ratio of transformation curve constant in the several adjusted conditions.

2. A series transformer having a primary winding of small potential drop adapted to be interposed in series relation to a distribution circuit, a secondary winding, and means for connecting a varying number of turns to assist or oppose to accommodate different secondary loads and maintain the ratio of transformation constant for any particular load on a permanently closed circuit.

3. A series transformer having a primary winding adapted to be connected in series re latlon to a distribution circuit, and a secondary winding having a main coil and a compensating coil of a variable number of turns in circuit with the main coil, whereby different secondary loads are accommodated and the ratio of transformation curve is rendered constant at the several loads.

4. A series transformer having a primary winding of low resistance for series connection in a load circuit, and a secondary winding adapted to be connected for boosting or bucking the secondary electromotive force in a permanently closed measuring circuit, whereby different loads may be accommodated and the ratio of transformation curve maintained constant.

5. A series transformer having a primary winding of low resistance for series connec tion in a load circuit, and a secondary winding composed of a generating coil and a compensating coil with changeable connections to permit. the compensating coil to assist or oppose in varying degrees the generating coil in a current measuring circuit, whereby different loads may be accommodated and the ratio of transformation maintained constant at any load.

6. A series transformer having a primary winding adapted to be connected in series relation to a supply circuit, a secondary coil of variable number of turns adapted to be connected in continuous closed relation to a load circuit of stable mine with different numbers of turns in series, whereby the ratio of transformation may be adjusted to maintain the accuracy of calibration of the load devices under varying operating conditions.

In testimony whereof I affix my signature, in presence of two witnesses.

GEORGE AUGUSTUS BURNHAM.

W'itncsses A. C -NELS0N, H. CALDER. 

